# std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept; | (1) | (since C++11)
bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept; | (2) | (since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj, std::memory_order order ) noexcept; | (3) | (since C++11)
bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj, std::memory_order order ) noexcept; | (4) | (since C++11)

Atomically changes the state of a std::atomic_flag pointed to by obj to set (true) and returns the value it held before.

1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.

### Parameters

obj | - | pointer to std::atomic_flag to access
order | - | the memory synchronization order

### Return value

The value previously held by the flag pointed to by obj.

### Notes

`std::atomic_flag_test_and_set` and `std::atomic_flag_test_and_set_explicit` can be implemented as obj->test_and_set() and obj->test_and_set(order) respectively.

### Example

A spinlock mutex can be implemented in userspace using an `std::atomic_flag`.

Run this code

    #include <atomic>
    #include <iostream>
    #include <thread>
    #include <vector>

    std::atomic_flag lock = ATOMIC_FLAG_INIT;

    void f(int n)
    {
        for (int cnt = 0; cnt < 100; ++cnt)
        {
            while (std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
                ; // spin until the lock is acquired
            std::cout << "Output from thread " << n << '\n';
            std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
        }
    }

    int main()
    {
        std::vector<std::thread> v;
        for (int n = 0; n < 10; ++n)
            v.emplace_back(f, n);
        for (auto& t : v)
            t.join();
    }

Output:

    Output from thread 2
    Output from thread 6
    Output from thread 7
    ...<exactly 1000 lines>...

### See also

atomic_flag (C++11) | the lock-free boolean atomic type (class)
atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) | atomically sets the value of the flag to false (function)
memory_order (C++11) | defines memory ordering constraints for the given atomic operation (enum)
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit

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